Since the announcement of the successful synthesis of high-aspect-ratio-few-walled boron nitride nanotubes (FW-BNNTs) in 1995, little progress had been made until very recently in the scale-up of their synthesis. In spite of the theoretical capabilities of FW-BNNTs to provide high strength-to weight, high temperature resistance, piezo actuation, and radiation shielding (via the boron content), the aerospace industry has had to rely on micron-sized graphite or boron fibers for structural applications. Further, despite their very desirable properties, neither FW-BNNTs nor single wall carbon nanotubes are used widely in aerospace manufacturing and similar industries, as industry is generally unwilling to pay the premium price for these high performance materials.
Prior to recent inventions of the present inventors, high-aspect ratio FW-BNNTs had only been produced in small amounts (from individual tubes to milligrams) by arc-discharge or laser heating methods. Further, these small amounts of FW-BNNTs were in the form of films not strands or fibers several centimeters in length. A separate class of boron nitride nanotubes known in the prior art has been produced by chemical vapor deposition or ball-milling. In ball-milling, finely-milled precursor powders of boron and catalyst are annealed in an N2 or ammonia gas atmosphere, sprouting nanostructures on their surfaces. (See Narottam P. Bansal, Janet B. Hurst, and Sung R. Choi, NASA/TM-2005-213874). In chemical vapor deposition, a Boron-containing vapor, for example B2O2, reacts with a nitrogen-containing gas, such as ammonia, for example, to deposit nanostructures on a substrate placed in a furnace. (See Chunyi Zhi, Yoshio Bando, Chengchun Tan and Dmitri Golberg, Solid State Communications, 2005, 135, 67.) These tubes are of large diameter, do not exhibit continuous crystalline sp2-type bonding structure which has drawn most theoretical interest, and are not strands or fibers.
The Inventors' recent work in the field of boron nitride nanotubes is described in three US. Patent Applications. Inventors' U.S. patent application Ser. No. 12/152,414 filed May 14, 2008 and incorporated herein by reference in its entirety describes a process for the production of at least centimeter-long boron nitride nanotube strands or fibers. Inventors' U.S. patent application Ser. No. 12/322,591 filed Feb. 4, 2009 and incorporated herein by reference in its entirety describes an apparatus for production of boron nitride nanotubes and a method of continuous removal of the formed boron nitride nanotubes from the synthesis chamber. Inventors' U.S. patent application Ser. No. 12/387,703 filed May 6, 2009 and incorporated herein by reference in its entirety describes a method for production of fibrils and yarns.
Boron nitride nanotubes and carbon nanotubes have several very different physical properties. Boron nitride nanotubes have electrical insulating properties while carbon nanotubes are valued for their electrical conducting properties. Additionally, carbon nanotubes and boron nitride nanotubes have different structural properties, with carbon having a propensity for closing across the end of the tube to form a “buckyball” type structure and boron nitride being more inclined to a tube structure.
Ma et al. (Syntheses and Properties of B—C—N and BN Nanostructures, Renzhi Ma, Dmitri Golberg, Yoshio Bando and Takayoshi Sasaki, Philosophical Transactions: Mathematical, Physical and Engineering Sciences, Vol. 362, No. 1823, Nanotechnology of Carbon and Related Materials (Oct. 15, 2004), pp. 2161-2186) describe multi-element B—C—N systems such as BN2, BC2N, BC3, and CNx. However these systems lack crystallinity and sp2 bonding.
Accordingly, a high-aspect ratio boron-carbon-nitrogen nanostructure with a high crystallinity would be highly desirable. Additionally, boron-carbon-nitrogen nanostructures are highly desirable as they are believed to have the potential to be useful semiconductors.